1. Field of the Invention
The present invention relates to a method for the continuous production of the time-release nitrogen fertilizer acetylene diurea.
2. The Prior Art
Fertilizers with a time-release effect have many advantages as compared with conventional mineral or organic fertilizers. They offer a delivery of nutrients to the plants that is better in keeping with the plants' needs, and thereby improve the utilization of the nutrients. This results in a reduction in nutrient losses, thereby reducing the burden on the environment and increasing the efficiency of fertilizer use. In addition, they make it possible to save work cycles and operating materials, and thereby agricultural costs.
A time-release effect of fertilizers can be achieved in different ways. One possibility is to surround granulated fertilizers that are easily soluble in water with a covering that is insoluble in water. The nutrient release from such coated fertilizers takes place with a delay, since the nutrients must first diffuse through the cover layer before they can be absorbed by the roots. Another possibility is to apply the nutrients in the form of chemical compounds in which they are not available to the plants at first. Only after a prior release step, for example chemical hydrolysis, enzymatic splitting and/or microbial transformation, has taken place, will the nutrients be available in a form that the plants, can utilize. Such fertilizers are also called chemical time-release fertilizers.
The idea of chemical time-release fertilizers dates back to the 19th century. At that time, Liebig in Germany and Murray in England suggested using nutrients in the form of salts with low solubility for plant fertilization.
Today, a large number of substances that contain nitrogen are produced and marketed as time-release fertilizers. The three most important ones, by far, are the condensation products of urea and formaldehyde, isobutyraldehyde, and acetaldehyde.
Other substances that are produced for use as fertilizers on a smaller scale are, among others, oxamide, acetylene diurea, melanine, substituted triazones, and the acetylene diurea shown below. 
Of these known time-release nitrogen fertilizers, acetylene diurea is of particular interest because of its comparatively high nitrogen content, its very good plant tolerance, as well as its excellent profile of effect. Its synthesis from urea and glyoxal has been described many times. L. Siemonsen reports on the production of acetylene diurea (glycoluril) from glyoxal and urea, with the addition of hydrochloric acid, in Ann. Chem. 333, (1904), pages 101 to 111.
The production of acetylene diurea (glycoluril) by means of condensation of glyoxal and urea is described in Chemistry and Industry, January 1979, pages 29 to 30 (Glycoluril as a slow-release nitrogen source for plants). For this purpose, solid urea was introduced into 40% glyoxal solution at 70° C., while stirring for 30 minutes. The resulting precipitated solid was filtered off, washed, and dried.
T. Shimizu describes the production of acetylene diurea by means of reacting glyoxal with urea in the presence of an acid as a catalyst in Soil. Sci. Plant. Nutr. 33 (1987, pages 291 to 298). To increase the yields, concentration series and variations in the catalyst were carried out. In addition, glyoxal that had not been reacted was passed back into the reaction. In this connection, the reaction was carried out discontinuously, with the glyoxal solution being metered into the urea solution drop by drop, in order to maximize the yield. The yield obtained was 87%. A temperature of 60° C. to 80° C., a reaction time of 1.5 to 3 hours, and a concentration of hydrochloric acid as the catalyst of 5 to 10% are indicated as being optimum reaction conditions. After six feed-back cycles, it was possible to increase the total yield to 91%.
U.S. Pat. No. 3,061,423 describes the use of acetylene diurea (glycoluril) as a fertilizer.
U.S. Pat. No. 2,731,472 relates to the production of heterocyclic compounds of glyoxal and urea, using acidic catalysis. Here, the molar ratio of urea to glyoxal is 2.01 to 2.30. The production takes place discontinuously, and it was possible to feed part of the reaction solution, from which the acetylene diurea was removed, back into the reactor. Before the acetylene diurea that was obtained was filtered off, the solution was neutralized with ammonia.
JP 2001 097974 relates to the production of acetylene diurea by means of the reaction of urea and glyoxal in the presence of hydrochloric acid as a catalyst.
JP 2000 264887 relates to the production of acetylene diurea by means of the reaction of urea and glyoxal, whereby a molar ratio of urea to glyoxal of 2.01 to 2.3 is also adjusted. The work is carried out with saturated urea solution.
JP 2000 290281 relates to the production of acetylene diurea by means of the reaction of urea and glyoxal, whereby urea and glyoxal are continuously metered into a saturated suspension of acetylene urea in a molar ratio of 2.01 to 2.30. Urea and glyoxal are reacted in aqueous solution, in the presence of acid catalyst that is continuously supplied. It is stated that the method allows the continuous production of acetylene diurea.
However, a disadvantage of the known methods is either a discontinuous conduct of the reaction, which is disadvantageous in the production of large amounts, or the use of a larger excess of urea, which must be removed and discarded in the final analysis, or the accumulation of larger amounts of by-products or mother liquor, which must be disposed of.